Process for electrodeposition of paint

ABSTRACT

In the electrodeposition of base solubilized, resinous materials from an aqueous dispersion onto a metal, anodic substrate immersed in the dispersion by passing a direct electrical current through the dispersion and substrate, generally speaking there are two stages in the deposition of the film. There is a first stage in which the current density increases due to a fixed or increasing voltage and relatively low film resistance; and a second stage in which the current density tends to decrease because of the increased resistance of the growing film. In accordance with our invention the overall deposition process is improved by maintaining the current density during the second stage of film deposition at a maximum value, predetermined by small scale experiments, by increasing the deposition voltage above the nominal rupture voltage of the film as required to maintain the relatively high current density value, which is a maximum value suitable for obtaining rapid film deposition and good film throw without damaging the film by boiling aqueous liquid or low boiling cosolvents therein.

United States Patent 1191 Campbell et al. I

[11 3,855,106 1451 Dec. 17, 1974 Appl. No.: 366,580

PROCESS FOR ELECTRODEPOSITION OF PAINT Inventors: Gregory A. Campbell,Romeo; I William B. Brown, Birmingham, both of Mich.

57 ABSTRACT materials from an aqueous dispersion onto a metal,anodic'substrate immersed in the'dispersion by passing substrate,generally speaking there are two stages in Assigneei Mntors Corporanon,the deposition of the film. There. is a first stage in Detron, Mlcnwhichthe current density increases due toa fixed or Filed: June 4, 1973increasing voltage and relatively low film resistance;

and a second stage in which the current density tends growing film. Inaccordance with our invention the [52] U.S.-Cl. 204/181 overalldeposition Process is improved by maintaining 51 Int. Cl C23b 13/00current density during the Second Stage of film [58] Field of Search204/181 Position at a'maximum value, predetermined by Small scaleexperiments, by increasing the deposition volt- [56] References Citedage above the nominal rupture voltage of the filmas required to maintainthe relatively high'current den- UNITED'STATES PATENTS sity value, whichis a maximum value suitable for ob- 3,355,374 I 11/1967 Brewer et al.204/l8l mining rapid film deposition and good film throw $323"; a Iwithout damaging the film by boiling aqueous liquid or 3:647:658 3/1972Hofling et a]. 204/l8l low bo'lmg cosolvems therem' 4 Claims, 3 DrawingFigures Primary Examiner-John H. Mack Assistant Examiner-Aaron Weisstuchv Attorney, Agent, or Fir mGeorge A. Grove 9) V o0 U a so 2 60 g 8 I :2E J '-40 3 g a: 7-20 0 1'0 4b 5b 6b 76 8b 9b I00 I no TIME, SECONDS Inthe electrodeposition of base solubilized, resinous a direct electricalcurrent through the dispersion and to decrease because of the increasedresistance of the mesa-.106

PATENTED DEC] H574 Q mmamiz Ht y 1 TIME SECONDS E om crcmzua ,hzwmmnu 5D N w, E 5 m m. 4 m 0 u. mm bfimnifi m w m m m .m n I rm t o, .0 I 8 mrm -w -w M -w -m m s 2 v o E F523 #2556 TIME, SECONDS PROCESS FORELECTRODEPOSITION OF PAINT This invention pertains to theelectrophoretic deposition of base solubilized, polycarboxylic acidresins on electrically conductive, anodic electrode substrates. Morespecifically, this invention pertains to a method of carrying out such apainting process to achieve better paint coverage without producingimperfections in the paint film, commonly known as rupture, by measuringthe current density particularlyduri'ng the later stages of the paintingof an article and maintaining the current density at a predeterminedmaximum value throughout the final buildup of the paint film to asuitable thickness.

The art of painting electrically conductive substrates byelectrophoretically depositing pigmented, base solubilized, syntheticresin onto the substrate is now widely practiced. One significantadvantage of such a painting process is that, in principle, a uniformfilm buildup is obtained over a substantial portion of the substrate.This is particularly important for corrosion resistance,

. 2 ing articles to be painted containing an aqueous dispersion ofsuitable base solubilized, polycarboxylic acid resin and suitablepigments, solvents and the like as may be required for a particularapplication. Either the painting tank is rendered cathodic in a directcurrent electrodeposition circuit or special cathodes are placed in thetank. The articles to be painted are immersed in the bath and renderedanodic and a direct current is a I voltage. Initially the currentdensity will increase rapfor example, on automobile car bodies. It isknown will rupture to produce holes or nonuniforrnities of thickness inthe paint film which either permit corrosion to take place or areunsightly, or both. The nominal rupture voltage varies for each resinsystem employed and, in fact, also is a function of the composition ofthe bath in which the resin is dispersed. Some resins have higherrupture voltages than others. Presently, high rupture voltages areconsidered to be in the range of 300 to 500 volts. However, it isdesired to' deposit the paint resin at still higher voltages or tootherwise obtain more complete coverage on a car body or other articlesfor purposes or corrosion resistance and the like. It is also desirableto accomplish the paint film buildup as rapidly as possible, consistentwith good quality paint film formation, so that the residence time ofthe article to be painted in the paint tank is minimized.

It is an object of the present invention to provide a method ofdepositing a base solubilized, polycarboxylic acid resin containingpaint on an anodic electrode substrate wherein the current densityduring the deposition, particularly in the final stages thereof, ismaximized so as to obtain rapid film buildup and throw, but to avoidfilm rupture.

It is a more specific object of the present invention to provide amethod of electrophoretically depositing suitable base solubilizedresins on an electrically conductive substrate wherein the depositionvoltage is increased toward the nominal rupture voltage as film buildupoccurs and, thereafter, the deposition voltage is further increased asnecessary to maintain a predetermined maximum current density which willmaximize the rate of paint film buildup and the throw of the filmwithout rupturing or otherwise damaging the film due to boiling of wateror cosolvent in the film.

In accordance with a preferred embodiment of our invention, these andother objects and advantages are accomplished by initially providing atank for immersidly, but as the electrical resistance of the growingfilm increases, the current density levels off and decreases unless thevoltage is further increased. Usually no film rupture occurs during thefirst stage of the deposition because the film is not thick enough toheat up and cause bath liquids entrained therein to boil. During thelast stage of the film deposition process we allow the current densityto fall to a predetermined maximum value and thereafter maintain currentdensity at said value for the rest of the deposition period byincreasing the deposition voltage as necessary. This maximum currentdensity value is determined experimentally as will be described in moredetail by conducting testson a laboratory scale. The maximum currentdensity is that value at which the film can be deposited without heatingthe film and the liquids entrained therein above the boiling point ofthe liquids. At such high current density and deposition voltage thefilm throw and rate of film buildup are maximized.

These and other objects and advantages tion will be more-fullyappreciated from a detailed description thereof which follows. In thedescription reference will be made to the drawings, in which:

FIG. 1 is a graph of current density and film temperature versus time ina specific electrophoretic painting experiment in which rupture occurredin the paint film;

FIG. 2 is a graph of current density and film temperain 'which filmrupture did not occur; and

FIG. 3 is a graph of current density and film temperature versus timedepicting another embodiment of our invention in an electrophoreticpainting process in which film rupture did notoccur.

During the electrodeposition of water-based paints,

v defects are encountered when film rupture occurs. This rupture hasbeen referred to as blowing or gassing of the coating and results whenthe local current density and film resistance combine to exceed acritical value. The film rupture produces a hole in the paint film orroughness due to differences in paint film thickness. Inelectrodepositing a particular resin composition at increasing voltageswithout interruption there is eventually reached a maximum voltage, therupture voltage, at which the described defects in the film occur. Bymeasuring and following both the temperature of the film as it isdeposited and the value of the current effecting the deposition we havefound that rupture is caused when the temperature of the film exceedsthe of our invenboiling point of the liquid entrained therein. Thisliquid typically comprises water and low boiling organic cosolventsdispersed therein. When the liquid boils, portions of the film areremoved. If the voltage is then quite high, substantial current flow maytake place to produce widespread destruction of the film. In ourexperience rupture usually does .not occur until a large percentage ofthe film has already been deposited.

In a commercial electrophoretic painting operation, of course, paintingbaths containing hundreds or even thousands of gallons of paint areemployed. During the painting of a single article neither thetemperature of the bath nor its resistance to electrical conductivitymay change appreciably. However, as the bath ages, its conductivity maychange (typically increases) due to ionic impurities which are carriedinto the bath or fonned therein. As the painting of a particular articlebegins, the initial film has little electrical resistance because it isvery thin. Therefore, there is little film heating due to PR losses. Themetallic substrate acts as a heat sink tocool the film. However, as thefilm increases in thickness, film heating increases and the film andliquids entrained therein can become quite hot in a short period of timeif the current flow is not controlled. Heretofore,, such control hasbeen accomplished by maintaining the deposition voltage below theso-called rupture voltage, or by interrupting the current flowaltogether so that the film has a chance to cool before further filmbuildup is attempted.

The following examples will illustrate how we have improved upon priorart electrophoretic painting processes to maximize film throw and rateof deposition while avoiding film rupture due to film overheating anboiling of liquids entrained therein.

A laboratory paint bath was prepared containing a commercial aminesolubilized, styrene-allyl alcoholbased alkyd resin body primerpigmented with iron' oxide particles. A conventional throw box wasassembled to serve as the article to be electrocoated. This devicecomprised two zinc phosphated, cold rolled steel panels, l8 inches by 4inches in dimensions. The edges of the panels were inserted into tworubber gaskets, each running the full length of the panels, so that thepanels were held spaced apart from each other by as inch in parallelrelationship. This assemblage of panels and rubber gaskets formed a box(throw box) which was open only at the top and the bottom. Athermocouple was applied to the outer surface of one of the panels forthe purpose of measuring the terrnperature of the film deposited on thepanel. The thermocouple as 0.015 inch in diameter and enclosed in astainless steel sheath. The throw box was immersed in the paint withonly the top portion extending above the bath level. An electric circuitwas prepared comprising a direct current potential source, anarnmeter,the phosphated steel panels as anodic substrates, the bath aselectrolyte and cathodic electrodes immersed in the 4 a minute. At thecompletion of the two minute painting operation the throw box wasremoved from the bath, disassembled and the panels examined. The outersurfaces of the panels were covered with the paint film. The paint filmhad also been formed on the inside surfaces of the panels to a distanceof ll inches up from the bottom edge of the box. The throw was-thus 11.A scratch was made on the panels throughthe paint film to the basemetal. The scratched panels were then sprayed for 336 hours with a saltsolution. Rusting was observed to have occurred at the. scratch, andrust formed a distance of 7/32 of an inch back under the paintedsurface. This indicated that the paint filmwas not fully continuous.Some porosity hadapparently been formed in the paint film due to ruptureduring the painting operation.

A second experiment, similar to that described above, was performedwithtwo new phosphated steel panels formed into a throw box, except thatin this case an uninterrupted,'constant deposition voltage of 400 voltsDC was imposed for 120 seconds. At the completion of the depositionprocess the panels were disassembled from the box and examined. A paintfilm throw of 12 inches was obtained which represented an increase of 1inch over that obtained at a constant deposition voltage of 350 voltsDC. However, the panels fared much worse in the salt spray scribe test.Corrosion was observed under the paint film a distance of 15/32 of aninch from the initial scribe. A substantial A third experiment wasperformed by following a practice in accordance with'our invention. Twonew phosphated, cold rolled steel panels were selected and formed into athrow box as described above. A paint bath of the above composition wasemployed. Electrophoretic deposition was commenced by imposing a voltageof volts DC for about 10 seconds. The current flow and film temperaturewere monitored during this experiment and the values'are summarized inFIG. 1. The current density is traced as the solid line and the filmtemperature as the broken line. At 50 volts the current densityinitially shot up to about 2% amperes bath spaced from the steel panels.The dimensions of the panels were such that the two external surfacesthereof made up a total of one square foot of surface area. Thus anammeter reading in amperes was in effect a current density reading inamperes/square foot. The rupture voltage of the paint was nominally 375volts.

A direct current voltage of 350 volts was impressed for 120 seconds. Theinitial current was over 10 amperes. However, it rapidly decreased tobelow one ampere within 20 seconds and below ampere in about persquarefoot and quickly'fell off in a period of about 10 seonds. to less thanone ampere per square foot. As the film thickness increased, so did itselectrical resistance. The film temperature at this time was of-theorder of 30C. At about 10 seconds from the commencement of painting, thedeposition voltage was abruptly increased to volts' DC. The current flowresponded immediately to a value of about 3% amperes per square foot andfell off during the next 10 seconds to about one ampere per square foot.After a total of about 20 seconds from the start of painting, thevoltage was increased to 350 volts. The current density in-v creased toa value just under 3 amperes per square foot and began to decrease. Atabout 50 seconds from the beginning of the painting operation thecurrent had decreased to about 0.8 ampere per square foot. From thispoint on the current density was maintained at 0.8 am-- pere per squarefoot by gradually further increasing the voltage as required. .Thetemperature of the film initially climbed to about 40 C. until theplating operation had continued for some 85 seconds. The filmtemperature then rather abruptly increased to over 100 C., at whichpoint liquids entrained in the film boiled and substantial film ruptureoccurred. In other words, one could not suitably deposit paint film ofthis composition at the current density of 0.8 ampere per square foot inthe terminal stage of painting without damaging the film producedthereby.

A fourth experiment was performed generally following theelectrodeposition procedure described with respect to the thirdexperiment above. The current density' and film'temperature weremeasured and recorded throughout the deposition. The respective valuesare summarized in the graph of FIG. 2, with the solid line representingthe current density and the broken line indicating film temperature. Asin the third experiment, the initial deposition voltage was 50 volts.This was increased to 150 volts after about seconds of film depositionand then to about 350 volts after an additional 10 seconds of filmdeposition. However, in this experiment the current density was allowedto fall, after the voltage increased to 350 volts, to a value of 0.55ampere per square foot. It was then necessary to increase the depositionvoltage above 350 volts to maintain the current density at 0.55 ampereper square foot. It is noted that the film temperature in this terminalstage of the deposition, from about 50 seconds on, remains below 40 C.

At the completion of the deposition the throw box was disassembled andthe two panels examined. The outer surfaces of the two panels were fullycoated. The inner surfaces of the panels were coated up from the bottomof the original box to a distance greater than the throw values obtainedin experiments 1 and 2 above. Furthermore, when the panels weresubjected to the salt spray scribe test as described above, no rust wasformed under the paint film adjacent the scratch mark. In other words,the salt spray scribe value was zero.

Thus it is seen that in electrocoating with a resin composition of thetype specifically herein employed one can advantageously obtain goodfilm throw and corrosion resistance by maintaining a current densityduring the last stage of film deposition of 0.55 ampere per square footor slightly above. However, if one maintains a current density of 0.8ampere per square foot during this period, film rupture occurs due toboiling of bath liquids in the deposited film. In accordance with ourinvention one would electropaint with described resin composition in aproduction scale operation at a current density of about 0.55 to 0.65ampere per square foot during the last half of the deposition to obtaingood film throw and corrosion resistance. As the production scale bathages itmay become contaminated with electrically conductive chemicalspecies and current densities in this range may then be maintained atslightly lower voltages. If one continued to employ the original highervoltages, film rupture could occur. However, by measuring and depositingat the predetermined current density one can safely obtain good filmcoverage without risk of film rupture.

In accordance with another embodiment of our invention the depositionvoltage can be gradually increased toward the nominal rupture voltage inthe first stage of deposition rather than increased stepwise asdescribed above. Current densities and film temperatures representativeof this procedure are depicted in -FIG. 3, with the solid linerepresentingcurrent density and the broken line representing filmtemperature. The deposition voltage is gradually increased to about 350volts (when using the resin composition identified in the aboveexperiments) during this first 20 to 30 seconds of film deposition. Withthe increasing voltage the rupture voltage has been reached, the currentdensity is permitted to decrease until it reaches the desiredpredetermined maximum current density desired for the final stage of thefilm deposition. At this point the deposition voltage is increased abovethe nominal rupture voltage as required to maintainthe desired maximumcurrent density for the second stage of the deposition.

In FIG. 3 this maximum current density was about 0.65 ampere per squarefoot. i

From the foregoing it will be appreciated that in general there are twosignificant discernible stages in they electrodeposition of waterdispersible paint films. In the first stage the voltage may be increasedstepwise or gradually, as desired, toward the nominal rupture voltageand the current density will increase and possibly intermittentlydecrease depending upon how the voltage increase is accomplished.Generally speaking, after about the first S0 to seconds of filmdeposition .the film thickness will have increased to the point at whichfurther increases in deposition voltage mustbe undertaken with caretoavoid film rupture. In accordance with our invention further voltageincreases are effected only to maintain a suitable predetermined maximumcurrent density consistent with the objectives of obtaining. maximumfilm throw and corrosion resistance and avoiding film rupture due toboiling of bath liquids entrained therein. Since the bath liquidsusually contain water and low boiling organic cosolvent liquidsdispersed therein, it is usually preferred to avoid heating the filmabove about C. In this'way the formation of small holes in the film dueto the evolution of low boiling organics can be avoided and a morecorrosion resistant film produced.

Organic resin coating materials which may be em- 5 Such materials mayinclude or be employed with other organic monomers and/or polymersincluding, but not by way of limitation, hydrocarbons and oxygensubstituted hydrocarbons, such as ethylene glycol, propylene glycol,glycerol, monohydric alcohols, carboxylic acids, ethers, aldehydes andketones. The film-forming material may include or be employed withpigments, metallic particles, dyes, drying oils, etc., and may bedispersed as a colloid, emulsion or emulsoid. Coating materials adaptedfor anodic deposition mayinclude one or more of the aforementionedresins having a plurality of free carboxyl groups or their equivalent intheir'polymeric structure. Dispersion of these resins in water can beefiected by the addition of a suitably basic material, such as ammonia,water soluble amines, mixtures of 7 monomeric and polymeric amines,potassium hydroxide and the like.

While our invention has been described in terms of a few specificembodiments thereof, it will be appreciated that other forms can readilybe adapted by one skilled in the art. Accordingly, our invention is tobe limited only by the following claims.

What is claimed is:

1. In the method of depositing a base solubilized, resinous,polycarboxylic acid material from an aqueous dispersion thereof onto anelectrically conductive, anodic substrate immersed in said dispersion bypassing a direct electric current through said dispersion and substratewhereby a resinous film of predetermined thickness is deposited on saidsubstrate, there being two discernible stages in the process ofdepositing said film: (a) a first stage in which the current densityincreases due to an initially fixed or increasing voltage, below thenominal rupture voltage of said material, and

relatively low film resistance and (b) a second stage in which the.current densty decreases to a low generally constant value because ofincreased resistance of the growing film, the improvement comprisingmaintaining said current density during said second stage of the filmdeposition process at a predetermined value greater than said low valueby increasing the deposition voltage to above said nominal rupturevoltage as required to maintain said current density value for a timesufficient to complete the film deposition, said predetermined currentdensity value being a maximum value found experimentally to be suitablefor obtaining maximum film throw without damaging the film by boilingaqueous or cosolvent liquid therein.

2. In the method of depositing a base solubilized, resinous,polycarboxylic acid material from an aqueous dispersion thereof onto anelectrically conductive, anodic substrate immersed in said dispersionbypassing a direct electric current through said dispersion andsubstrate whereby a resinous film of predetermined thickness isdeposited on said substrate, there being two discernible stages in theprocess of depositing said film: (a) a first stage in which the currentdensity in creases due to an initially fixed orincreasing voltage, belowthe nominalrupture voltage of said material, and relatively low filmresistance and (b) a second stage in which the current density decreasesto a low generally constant value because of increased resistance of thegrowing film, the improvement comprising maintaining said currentdensity during said second stage of the film deposition process at apredetermined value greater than said low value by increas-- tally to besuitable for obtaining maximum film throw without heating the depositedresin film above C. a

3. A method of depositing a base solubilized, resinous, polycarboxylicacid material from an aqueous dispersion thereof onto an electricallyconductive anodic substrate,

. said resin having a predetermined,

voltage, comprising immersing a said substrate in-the aqueous resin dis-1 persion,

passing a direct electric current through said dispersion and substrateat a first voltage value below said nominal rupture voltage for a firstperiod of seconds to commence deposition of a resin film on saidsubstrate, the current density initially increasing abruptly andthereafter gradually decreasing,

increasing the deposition voltage toa second value greater than saidfirst value but less than said nominal rupture voltage and continuing todeposit said resin at said voltage for a second period of seconds, thecurrent density again increasing abruptly and thereafter graduallydecreasing,

increasing the deposition voltage to about said nominal rupture voltagewhile said current density abruptly increases and thereafter graduallydecreasing,

and then maintaining said current density at a predetermined value byfurther increasing said voltage to complete the deposition of a resinfilm of predetermined thickness on said substrate, said predeterminedcurrent density value being a maximum value found experimentally to besuitable for obtaining maximum film throw without damaging said film byboiling aqueous or cosolvent liquid therein.

nominal rupture 4. A method of depositing a base solubilized, resinous,polycarboxylic acid material from an aqueous dispersion thereof onto anelectrically conductive anodic substrate, said resin having apredetermined, nominal rupture voltage, comprising immersing a saidsubstrate in the aqueous resin solution, passing a direct electriccurrent through said dispersion and substrate by employing an initialdeposition voltage below said nominal rupture volt-' age and thereafterincreasing said deposition voltage toward said nominal rupture voltageto effect deposition of a resin film on said substrate, the currentdensity initially increasing and thereafter gradually decreasing,

and then maintaining said current density at a predetemrined value byfurther increasing said voltage to complete the deposition of a resinfilm of predetermined thickness on said substrate, said predeterminedcurrent density value being a maximum value found experimentally to besuitable for obtaining maximum film throw without damaging said film byboiling aqueous or cosolvent liquid therein.

1. IN THE METHOD OF DEPOSITING A BASE SOLUBILIZED, RESINOUS,POLYCARBOXYLIC ACID MATERIAL FROM AN AQUOUS DISPERSION THEREOF ONTO ANELECTRICALLY CONDUCTIVE, ANODIC SUBSTRATE IMMERSED IN SAID DISPERSION BYPASSING A DIRECT ELECTRIC CURRENT THROUGH SAID DISPERSION AND SUBSTRATEWHEREBY A RESINOUS FILM OF PREDETERMINED THICKNESS IS DEPOSITED ON SAIDSUBSTRATE, THERE BEING TWO DISCERNIBLE STAGES IN THE PROCESS OFDEPOSITING SAID FILM; (A) A FIRST STAGE IN WHICH THE CURRENT DENSITYINCREASES DUE TO AN INITIALLY FIXED OR INCREASIBG VOLTAGE, BELOW THENOMINAL RUPTURE VOLTAGE OF SAID MATERIAL, AND RELATIVELY LOW FILMRESISTANCE AND (B) A SECOND STAGE IN WHICH THE CURRENT DENSITY DECREASESTO A LOW GENERALLY CONSTANT VALUE BECAUSE OF INCREASED RESISTANCE OF THEGROWINNG FILM, THE IMPROVEMENT COMPRISING MAINTAINING SAID CURRENTDENSITY DURING SAID SECOND STAGE OF THE FILM DEPOSITION PROCESS AT APREDETERMINED VALUE GREATER THAN SAID LOW VALUE BY INCREASING THEDEPOSITION VOLTAGE TO ABOVE SAIDC NOMIAL RUPTURE VOLTAGE AS REQUIRED TOMAINTAIN SAID CURRENT DENSITY VALUE FOR A TIME SUFFICIENT TO COMPLETETHE FILM DEPOSITION, SAID PREDETERMINED CURRENT DENSITY VALUE BEING AMAXIMUN VALUE FOUND EXPERIMENTALLY TO BE SUITABLE FOR OBTAINING MAXIMUMFILM THRWO WITHOUT DAMAGING THE FILM BY BOILING AQUEOUS OR COSOLVENTLIQUID THEREIN.
 2. In the method of depositing a base solubilized,resinous, polycarboxylic acid material from an aqueous dispersionthereof onto an electrically conductive, anodic substrate immersed insaid dispersion by passing a direct electric current through saiddispersion and substrate whereby a resinous film of predeterminedthickness is deposited on said substrate, there being two discerniblestages in the process of depositing said film: (a) a first stage inwhich the current density increases due to an initially fixed orincreasing voltage, below the nominal rupture voltage of said material,and relatively low film resistance and (b) a second stage in which thecurrent density decreases to a low generally constant value because ofincreased resistance of the growing film, the improvement comprisingmaintaining said current density during said second stage of the filmdeposition process at a predetermined value greater than said low valueby increasing the deposition voltage to above said nominal rupturevoltage as required to maintain said current density value for a timesufficient to complete the film deposition, said predetermined currentdensity value being that value found experimentally to be suitable forobtaining maximum film throw without heating the deposited resin filmabove 80* C.
 3. A method of depositing a base solubilized, resinous,polycarboxylic acid material from an aqueous dispersion thereof onto anelectrically conductive anodic substrate, said resin having apredetermined, nominal rupture voltage, comprising immersing a saidsubstrate in the aqueous resin dispersion, passing a direct electriccurrent through said dispersion and substrate at a first voltage valuebelow said nominal rupture voltage for a first period of seconds tocommence deposition of a resin film on said substrate, the currentdensity initially increasing abruptly and thereafter graduallydecreasing, increasing the deposition voltage to a second value greaterthan said first value but less than said nominal rupture voltage andcontinuing to deposit said resin at said voltage for a second period ofseconds, the current density again increasing abruptly and thereaftergradually decreasing, increasing the deposition voltage to about saidnominal rupture voltage while said current density abruptly increasesand thereafter gradually decreasing, and then maintaining said currentdensity at a predetermined value by further increasing said voltage tocomplete the deposition of a resin film of predetermined thickness onsaid substrate, said predetermined current density value being a maximumvalue found experimentally to be suitable for obtaining maximum filmthrow without damaging said film by boiling aqueous or cosolvent liquidtherein.
 4. A method of depositing a base solubilized, resinous,polycarboxylic acid material from an aqueous dispersion thereof onto anelectrically conductive anodic substrate, said resin having apredetermined, nominal rupture voltage, comprising immersing a saidsubstrate in the aqueous resin solution, passing a direct electriccurrent through said dispersion and substrate by employing an initialdeposition voltage below said nominal rupture voltage and thereafterincreasing said deposition voltage toward said nominal rupture voltageto effect deposition of a resin film on said substrate, the currentdensity initially increasing and thereafter gradually decreasing, andthen maintaining said current density at a predetermined value byfurther increasing said voltage to complete the deposition of a resinfilm of predetermined thickness on said substrate, said predeterminedcurrent density value being a maximum value found experimentally to besuitable for obtaining maximum film throw without damaging said film byboiling aqueous or cosolvent liquid therein.